A radiation detector for detecting radiation such as X-rays includes a radiation detecting element like, for example, SDD (Silicon Drift Detector). The radiation detecting element outputs a charge signal in accordance with detected radiation. The charge signal is converted into a voltage signal by a preamplifier. Signal processing including spectrum generation or the like is performed based on the voltage signal.
FIG. 1 is a circuit diagram of a preamplifier in the conventional radiation detector. A FET (Field Effect Transistor) 72 is connected at a stage preceding an amplifying circuit 71. A signal from the radiation detecting element is input to the amplifying circuit 71 through the FET 72. Moreover, each of a feedback capacitor 73 and a reset switch 74 is connected to be in parallel with the amplifying circuit 71 and FET 72. There are cases where the amplifying circuit 71 is configured with an operational amplifier and a transistor, where the amplifying circuit 71 is configured as an integrated circuit, and where the amplifying circuit 71 and FET 72 are configured as one integrated circuit.
FIG. 2 is a schematic plan view illustrating a part of the structure of a conventional preamplifier formed as a chip. FIG. 3 is a schematic view illustrating a section of a part of the conventional preamplifier formed as a chip. A bonding pad for input is denoted by a reference number 74 in the drawings. The bonding pad 74 is connected to a bonding wire 76 further connected to a radiation detecting element. In the preamplifier, a metal wiring 75 is connected to the bonding pad 74 through an interconnection layer arranged below the bonding pad 74, while the feedback capacitor 73 is connected to the metal wiring 75. An electrode 731 at the upper side of the capacitor 73 is connected to the metal wiring 75 while an electrode 732 at the lower side thereof is connected to an output terminal of the preamplifier. The electrode 731 at the upper side of the capacitor 73 is also connected to the FET 72 through the metal wiring 75. The components in the preamplifier are formed on a silicon (Si) substrate 77 while a part excluding the upper surface of the bonding pad 74 is covered with an oxide film (not shown). Thus, the conventional preamplifier is formed as an integrated circuit. Japanese Patent Application Laid-Open No. 2000-58765 discloses an example of an integrated circuit including a bonding pad and a capacitor.